Study On Gasification Characteristics And Kinetics Of Coal/biomass Char In CO₂/H₂O Atmosphere

Gasification technology is an important way for coal/biomass energy utilization and a key technology to achieve clean energy utilization and transformation.Compared with pyrolysis,char gasification is the rate-limiting step of the entire process,which affects the design and operation of the gasifier.Char gasification behavior and char characteristics(pore structure,metal oxide,carbon structure,etc.),reaction conditions(temperature,atmosphere and partial pressure,etc.),test methods(isothermal method,non-isothermal method)and instruments(fixed bed,Fluidized bed,etc.)are closely related.Among them,the influence of the types and contributions of metal oxides in char on gasification is still unclear.The difference between isothermal and non-isothermal determinations remains to be revealed.Isothermal testing under water vapor atmosphere is still limited by the reaction analyzer.Based on this,this study used typical coal/biomass char to study:(1)The gasification behavior of distiller's grains pyrolyzed with different metal oxides in the thermal gravimetric analyzer(TGA)under isothermal and non-isothermal conditions,and the kinetics are compared;(2)The difference in the gasification reaction behavior of char-CO₂ in TGA and micro fluidized bed reaction analyzer(MFBRA)was compared,and the difference between the two types of instruments was revealed;(3)The reaction behavior and kinetics of char under CO₂ and H₂O atmosphere were investigated by using MFBRA with high heating rate.The experiment content and results are as follows:(1)Based on the idea of decoupling gasification to prepare distiller's grain char,the fixed-bed TGA system compares deashing char(ALC)and metal oxide char(ALC-K₂O,ALC-Na₂O,ALC-Ca O,ALC-Fe₂O₃ and ALC-Mg O)isothermal and non-isothermal gasification characteristics,using distributed activation energy model(DAEM)and Flynn-Wall-Ozawa(FWO)conversion rate method to calculate the kinetics.The results show that the catalytic effects of metal oxides on char are significantly different,and their gasification activity is in order:ALC-K₂O>ALC-Na₂O>ALC-Ca O>ALC-Fe₂O₃>ALC-Mg O>ALC.The kinetic analysis found that the activation energy under isothermal gasification is less than the activation energy under non-isothermal,indicating that the higher energy utilization rate of isothermal gasification in the entire gasification reaction process is more suitable for guiding practical industrial applications.At the same time,the activation energy data obtained by the two analysis methods have the same change trend:ALC-K₂O<ALC-Na₂O<ALC-Ca O<ALC-Fe₂O₃<ALC-Mg O<ALC,which is consistent with the law of char gasification activity.Further verify the accuracy of the experimental results.(2)Based on the idea of decoupling gasification to prepare Naomaohu coal char,after steam activation treatment,the structural characteristics of char(UC,AC)before and after activation were characterized,and the analysis found that AC has a large specific surface area,strong CO₂ adsorption capacity,pore volume,number and reactive sites.Comparing the isothermal gasification characteristics and kinetic behavior of AC under MFBRA and TGA,it is found that the reaction rate is higher when the gasification reaction is carried out by MFBRA,and the activation energy(apparent activation energy)calculated according to the equal conversion rate method is larger,indicating the use MFBRA has better heat and mass transfer advantages during gasification reaction and effectively reduces the inhibitory effect brought by the gas-solid diffusion process.(3)The influence of ash,different CO₂ and H₂O gasification agent partial pressures on the gasification performance of char was investigated in MFBRA.It is found that the catalytic effect of minerals in the ash on the gasification is much greater than the effect of deashing on the structure of char.Comparing the experimental results under different gasification agent partial pressures(10%,20%,30%,40%),it is found that compared with CO₂ gasification,H₂O gasification reaction time is short,the reaction rate is large,the gas product is more,and the activation energy is small,indicating that coal char has better gasification performance in H₂O atmosphere.Finally,the activation energy obtained under the same atmosphere and different partial pressures is basically unchanged,which shows that MFBRA has higher adaptability to both CO₂ and H₂O atmospheres,and the obtained kinetic data is more in line with the intrinsic kinetic behavior.